Baking form and method of making same

ABSTRACT

An aluminum baking form is provided with internal and external coatings of aluminum oxide which are of black coloration and which contain metals other than aluminum and serve as infrared transmitters or collectors. The metal incorporated in the aluminum oxide layer is preferably tin or silver or both.

FIELD OF THE INVENTION

The present invention relates to improved aluminum utensils for bakingpurposes and, more particularly, to aluminum baking forms. The inventionespecially relates to improved aluminum baking forms having internal andexternal aluminum oxide coatings of black coloration, and to the use ofspecial aluminum oxide coatings for aluminum utensils, especially bakingforms.

BACKGROUND OF THE INVENTION

It is known to provide cooking utensils, especially baking dishes,trays, forms and the like with various coatings for different purposes,e.g. as non-stick coatings, as heat absorbers or transmitters, and asthermal conductors.

For the purposes of this discussion of the background of the inventionand for the subsequent discussion of the invention itself, the term"baking form" will be used to describe any utensils having a recess inwhich a comestible may be baked. Typical of such "baking forms" are piepans, cake pans, ring pans and the like.

The term "aluminum" as used herein to refer to baking forms and likeutensils and as it will be used hereinafter in connection with thediscussion of the invention, is intended to include all conventionalaluminum alloys which have been hitherto used for the production ofbaking utensils and existing alloys suitable for use in baking utensilswhether or not they have been so used heretofore.

Baking utensils, especially baking forms of the type described, aregenerally fabricated from sheet aluminum by deep drawing. The sheetmaterial has also been constituted of aluminum sheet or stripheretofore. The only requisite has been that the metal from which thebaking form was drawn have deep-drawing qualities.

As long as the baking form is constituted of steel sheet, it isnecessary to provide corrosion protection against rusting of the innerand outer surfaces.

To this end, anticorrosion coatings are applied internally andexternally and it has been found that tin coating (to produce tin plate)or, more recently, coatings of polytetrafluoroethylene are satisfactoryfor this purpose.

Coatings of polytetrafluoroethylene, however, suffer from poor adhesionto the substrate and reduce the heat transfer rather than improve thesame. As a result, it is difficult to brown the comestibles which aresubject to baking. Browning thus either never occurs or is effected onlyat high baking temperatures which may be detrimental to the coating.

The heat transfer is primarily by heat conduction and experience hasshown that infrared radiation is scarcely capable of being passedthrough the walls of the baking form.

The "browning" of the baked material is, however, effected principallyby the infrared radiation.

The foregoing applies to baking forms which consist of aluminum sheetprovided internally and externally with colorless aluminum-oxidecoatings and/or with polytetrafluoroethylene.

It has been found to be possible, in prior work in this field, toimprove the heat transfer of the baking forms to the comestible materialto be baked by forming the baking utensil from copper sheet. In fact,baking utensils have also been formed from steel sheet provided with abrownish coating. Neither technique has been fully successful because ofthe problem of corrosion. The latter techniques have never influencedthe aluminum-sheet technology in dealing with aluminum baking utensilsand, as far as I am aware, there has been no fully successful attempt toimprove the heat transfer of aluminum-sheet baking utensils to thecomestible product.

OBJECTS OF THE INVENTION

It is the principal object of the present invention to eliminate theaforedescribed disadvantages and provide an improved baking form fromaluminum sheet material which is characterized by high heat transfer andthe capability to enable, without the use of destructive temperatures,satisfactory browning of the comestible product.

Another object of the invention is to provide an improved aluminumbaking form with high heat transfer and low corrodibility and of lowcost.

Still another object of the invention is to provide a baking form fromaluminum aheet material whereby the disadvantages or earlier forms areavoided and high heat transfer is obtained with low corrodibility.

Still another object of the invention is to provide a new use for acertain type of aluminum sheet material.

SUMMARY OF THE INVENTION

These objects and others which will become apparent hereinafter areattained, in accordance with the present invention, in a baking formwhich is composed of aluminum sheet material and is coated internallyand externally with an aluminum oxide layer colored black by theincorporation therein of at least one metal other than the aluminum. Thelayer serves as an infrared transmitter and absorber and thus the outercoating of the utensil picks up incident infrared radiation and theinner layer acts as a transmitter for this radiation to the comestiblematerial. The metal which is incorporated in the aluminum oxide layer toblacken the latter is preferably tin or silver.

The black aluminum oxide coating provided internally and/or externallycan be provided by the following steps:

(a) etching or pickling in sodium hydroxide solution (aqueous),

(b) pickling in nitric acid,

(c) anodization by the direct-current sulfuric acid method, and

(d) electrolytic coloring with the incorporation of tin compounds and/orsilver compounds, preferably tin sulfate, in the electrolyte contactingthe surfaces to be colored and coated.

At least the electrolytic coloring and anodic processes are carried outon the substrate and preferably all of the processes (a) through (d)thereof are carried out in the order described.

The electrolytic coloring can, however, also be carried out toincorporate other metals besides tin or silver in the aluminum oxidecoating. In any case, an aluminum oxide coating is first orsimultaneously formed upon the sheet and the metal (coloring metal) isincorporated in the lattice structure of the aluminum oxide coating.

The etching or pickling is preferably carried out in a sodium hydroxidesolution which may have a sodium hydroxide concentration between 2% and20%, preferably between 5% and 12% and most advantageously about 9%. Thetemperature for this etching or pickling step is preferably between 20°C. and the boiling point of the solution and most advantageously between50° C. and 70° C. with best results being obtained at 65° C.

The acid pickling step is effected at a temperature between 10° C. and30° C., preferably ambient or room temperature of about 20° C., insemiconcentrated sulfuric acid.

The anodization step is preferably carried out between 5° C. and 40° C.,most advantageously between 10° C. and 30° C. and with best results atabout 20° C. with a current density of about 1.5 amperes per decimeter².The current density may, however, range between 1 and 2 amperes perdecimeter² and even lie between 0.25 and 10 amperes per decimeter².

The electrolytic coloring step is, however, of greatest importance tothe present invention and is preferably carried out by alternatingcurrent at line frequency (50 or 60 Hz) transformed from line voltage toa voltage of 15 to 20 volts and at a temperature of about 22° C. Thetemperature can be varied between 15° C. and 30° C. The coloring bathcomposition for 100 liters (best mode) is preferably

6.50 kg zinc sulfate,

6.00 kg of polyglycol,

10.00 kg of 65% phenylsulfonic acid,

10.00 kg of concentrated sulfuric acid,

2.00 kg of tartaric acid, and

0.50 kg of aluminum sulfate,

When silver is to be incorporated into the system, it may be used in thesame quantity as the zinc sulfate, advantageously in the form of thesilver nitrate, or the zinc sulfate component may be replaced by amixture of zinc sulfate and silver nitrate.

Each of the values given in the foregoing Table may be varied between±25%.

Following the electrolytic coloring step, the coatings are subjected toconsolidation in de-ionized water at a temperature of 98° C. to 100° C.or in superheated steam. The treatment time for the individual steps maybe selected at will as long as the desired degree of black coloration isobtained. As a practical matter, each step can be carried out for aperiod between 5 minutes and 3 hours without difficulty although forbest results, the various treatments should take no longer than 15minutes to 1 hour.

Naturally, the baking form, after shaping from aluminum sheet, should besubjected prior to the electrolytic treatment to cleaning anddegreasing. This can be effected in the usual way, e.g. by alkalinepickling.

The invention is based upon the surprising discovery that a baking formwhich is internally and externally provided with a black coating andanodically in the aforedescribed manner functions in a unique way duringthe heat transfer from the exterior to the comestible material withinthe baking form.

The external coating appears to act as an infrared absorber so thatimpinging infrared radiation, e.g. from the heat of the baking oven, isnot reflected but rather is practically fully absorbed. The resultingheat generated in the aluminum wall of the baking utensil is transformedinto infrared radiation which is re-emitted and thus impinges upon thecomestibles to be baked.

The coating thus constitutes an infrared absorber on the one hand and aninfrared transmitter on the other.

Even more surprisingly the coating is not only significant because ofits effect in the heat transfer, but also because it providespractically complete protection against corrosion. The coating is ableto withstand substantially all mechanical stresses the baking utensilmay be subjected to.

The coatings have no effect upon the comestible product and are notcapable of introducing health-affecting substances into the comestiblesbaked in the utensil. The baked material does not adhere strongly ordetrimentally to the coating and the coatings can be applied withoutparticular difficulty to all aluminum alloys now in use for thefabrication of baking forms.

An even greater advantage of the system of the present invention mayreside in the fact that the baking time is significantly reduced,thereby resulting in energy saving.

Stated otherwise, for a given baking time, the baking temperature can bereduced for a given result and, conversely, the usual baking temperaturemay be applied and the baking time thereby reduced. The system of thepresent invention has been found to allow intensive browning of thebaked material without significant danger of burning.

BRIEF DESCRIPTION OF THE DRAWING

The above and other objects, features and advantages of the presentinvention will become more readily apparent from the followingdescription, reference being made to the accompanying drawing in which:

FIG. 1 is a cross-sectional view through a ring-type baking formaccording to the present invention; and

FIG. 2 is a detail view of the region II of FIG. 1, drawn to a greatlyenlarged scale.

SPECIFIC DESCRIPTION

A conventional one-piece ring-type baking form has been shown in FIGS. 1and 2 and is composed of aluminum sheet 1 by the deep drawing process.

The aluminum sheet 1 is formed internally and externally with respectivecoatings 2 which are applied anodically and consist of aluminum oxide.These aluminum oxide coatings are colored black and the coloring step iscarried out as described above. These coatings thus function as infraredabsorbers and transmitters in the manner previously described.

In FIG. 1, the undulating arrows 3 represent the impinging infraredradiation and the infrared radiation which is radiated from the innercoating inwardly to the material to be baked.

SPECIFIC EXAMPLE

A baking form which satisfies all of the requirements set forth aboveand has all the advantages described, is produced in the usual manner bydeep drawing and aluminum sheet. The coating is applied as follows:

(a) The form is immersed in 9% sodium hydroxide solution (aqueous) at65° C. for a period of 1 minute for etching and pickling.

(b) The thus-etched and pickled baking form is immersed insemiconcentrated nitric acid at a temperature of 20° C. for 10 seconds,thereby achieving a certain degree of neutralization and furtherpickling.

(c) The form is subjected to internal and external anodization againstinternal and external electrodes over a period of 40 minutes by thedirect current sulfuric acid anodization process using a current densityof 1.5 amperes per decimeter² and a temperature of 20° C.

(d) The utensil, coated with aluminum oxide layers on both surfaces, isthen colored for a period of 8 to 10 minutes with transformer-producedalternating current (50 Hz) at 15 to 20 volts and a temperature of 22°C. The best results were obtained with 18 to 20 volts.

The bath composition for the electrolytic coloring step is as followsfor 100 liters:

6.5 kg zinc sulfate,

6.0 kg polyglycol,

10.0 kg of 65% phenylsulfonic acid,

10.0 kg concentrated sulfuric acid,

2.0 kg tartaric acid, and

0.5 kg aluminum sulfate.

(e) The block-colored coatings are consolidated by treating the utensilwith de-ionized water at a temperature of 98° C. to 100° C. for a periodof 60 minutes. A similar result was obtained by subjecting the coatingsto superheated steam for a similar period.

Similar results were also obtained when 6.5 kg of silver nitrate wassubstituted for the 6.5 kg of zinc sulfate and when, instead of the 6.5kg of zinc sulfate, 3.25 kg each of zinc sulfate and silver nitrate wereused in the coloring bath.

I claim:
 1. A baking form consisting of aluminum sheet and havingexternal and internal surfaces provided with respective aluminum oxidecoatings formed unitarily with the sheet and colored black by theincorporation therein of a coloring metal and constituting an infraredabsorber and an infrared transmitter, respectively.
 2. The baking formdefined in claim 1 wherein the metal incorporated in the aluminum oxidelayers is selected from the group which consists of tin and silver. 3.The baking form defined in claim 1 wherein both tin and silver areincorporated in each of said aluminum oxide layers.
 4. The use ofaluminum sheet coated on opposite surfaces with aluminum oxide andcolored black by the incorporation of metals therein in baking formsaccording to claim 1.